The present invention relates to an image pickup apparatus for an endoscope, and more particularly, to an apparatus for producing an image of an object to be observed under illumination of light emitted from an illumination optical system of the endoscope employing a solid-state image pickup device.
In recent years, an electronic type endoscope which produces an image of an object to be observed employing a solid-state image pickup device and displays the image of the object on a display apparatus such as a Brown tube has been brought into practical use.
The known solid-state image pickup device adapted to be used in an endoscope include charge transfer device such as a charge coupled device (CCD), a bucket brigade device (BBD) and a charge priming device (CPD), a static induction transistor (SIT) type and a metal oxide semiconductor (MOS) type.
Among the advantages of electronic type endoscope employing a solid-state image pickup device are that they make it easy to record an image of an object to be observed and are small (and will be miniaturized increasingly in future due to advances in high integration techniques) in comparison with an endoscope in which an optical image is formed on a fiber bundle for guiding an optical image. However, when the solid-state image pickup device is employed, it is necessary to form a color image.
A white color illumination lamp generally in use radiates more energy in an infrared ray wavelength zone than in a visual ray wavelength zone and the spectral properties of an illumination optical system and an image pickup optical system including all elements such as a lens system, a light guide and a solid-state image pickup device are not uniform so that it is difficult to reproduce a faithful color image. In addition, it is necessary to detect a difference in color tones in order to discriminate between an affected part and a normal part in a coeliac cavity. It is known that when an inspection is made with an endoscope the clear contrast has been formed by arranging a color filter of the blue group in an eyepiece portion of the endoscope. For example, it is possible to detect a small cancer.